Natural rubber is used in various fields, including industrial products such as tires, belts and rolls, and sports products such as tennis balls. Rubber products are generally subjected to compression-recovery-elongation repeatedly during use, which results in accumulation of loss energy to generate heat. This heat promotes rubber fatigue and thus contributes to shortened life of the rubber products. The larger the amount of the loss energy represented by loss tangent, tan δ, is, the more the heat build-up is. Further, the rolling resistance of a tire is empirically known to be dependent on the value of tan δ at 50° C. to 70° C. This means that there are problems that a high tan δ causes an increase not only in heat build-up but also in rolling resistance, and therefore causes a decrease in fuel economy of vehicles. Accordingly, it is desirable for treads of fuel-saving tires to have a low tan δ at 50° C. to 70° C.
Some documents disclose methods for reducing the protein and gel contents in natural rubber to reduce the tan δ of a rubber composition. For example, Patent Literature 1 discloses a method of immersing solid natural rubber swollen with a solvent in an alkali hydroxide solution. Patent Literature 2 discloses a method of removing magnesium phosphate from natural rubber latex by adding a phosphate to the latex. Patent Literature 3 discloses a method of adding a proteolytic enzyme and a surfactant to natural rubber latex and aging the latex. Patent Literature 4 discloses a method of adding a surfactant to natural rubber latex and performing a washing treatment.
However, these methods can reduce the protein and gel contents to some extent, but not yet to a sufficient level. In general, deproteinization can reduce the protein content, but disadvantageously it cannot sufficiently remove, particularly, phospholipids which are thought to be one of the factors for formation of a gel fraction in natural rubber.
In addition, these methods can reduce the protein and gel contents to some extent, but not yet to a sufficient level, and further reduction in tan δ is thus desired. Tires are also especially desired to have durability such as abrasion resistance, and it is commonly hard to achieve good fuel economy and durability at the same time.